1. Field of the Invention
The present invention relates to an optical fiber mode-locked laser to produce an ultrashort pulse, and more particularly to an optical fiber laser simplifying the structure by using a cholesteric liquid crystal circular polarization mirror and able to lower the threshold intensity of pump light required for mode locking operation.
2. Information Disclosure Statement
The ultrashort pulse originated from an optical fiber laser is one of the spotlighted light sources able to be used for high-speed optical communication due to its short pulse duration. The most effective method to produce an ultrashort pulse is mode-locking operation. Conventional mode-locked optical fiber laser is mostly achieved under figure-8 type or ring type resonator structure, and it can also be accomplished under the linear structure.
FIG. 1 is a view illustrating a structure of a conventional figure-8 type optical fiber mode-locked laser, FIG. 2 is a view illustrating a structure of a conventional ring type optical fiber mode-locked laser, and FIG. 3 is a view illustrating a structure of a conventional linear optical fiber mode-locked laser. In FIGS. 1, 2 and 3, the numeric 11 represents a (50:50) optical fiber directional coupler, 12 represents a wavelength division optical fiber coupler, 13 represents a (90:10) optical fiber directional coupler, 21 and 22 represent polarization controllers, 31 represents an optical isolator, 41 represents an erbium doped optical fiber, 42 represents a dispersion shifted fiber, 51 represents a light pumping laser diode, 61 represents a laser resonator mirror, 62 represents a laser output mirror, and 82 represents a linear polarizer respectively.
As mentioned above, in the conventional optical fiber laser, a mode-locked optical fiber laser is mostly achieved under figure-8 type (FIG. 1) or ring type (FIG. 2) structure. However, these methods have low efficiency and low economic competence since they have complex structures, need expensive optical parts, and losses are comparably high inside the resonator such as optical isolator. In the case of linear optical fiber mode-locked laser (FIG. 3), which does not need an optical isolator, however, it has a defect that the intensity of the pump light is relatively high to oscillate mode-locking operation due to the spatial hole burning by the formation of standing wave.
It is therefore the object of the present invention to provide an optical fiber laser to produce an ultrashort pulse, which is able to lower the threshold intensity of the pump light required for mode-locking operation and has simple structure by introducing cholesteric liquid crystal circular polarization mirror.
To achieve the object, the present invention comprises a gain medium doped optical fiber to achieve a population inversion between high and low energy levels by a pump light source and to oscillate optical waves in sequence, a cholesteric liquid crystal circular polarization mirror to transmit the waves from the pump light source and to be operated as a circular polarization reflection mirror for the oscillated optical waves from the gain medium doped optical fiber, a polarization controller to control the polarization state of the proceeding light which is oscillated from the gain medium doped optical fiber, a dispersion shifted fiber to give a non-linear effect to the proceeding light through the polarization controller, and an optical fiber laser output mirror which forms a resonator by putting out some portion of the proceeding light by transmission and reflecting the other portion of the light.